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Previous NIOSH studies indicate that approximately 90% of evaluated recreational boats produced potentially hazardous CO concentrations. When the boats were tested during previous NIOSH studies under stationary conditions, the CO concentrations were high at the stern (500 ppm to 1000 ppm). In addition, cabin cruiser measurements ranged from 800 ppm to 1000 ppm on the lower deck (less than 15 ppm in the interior). Carbon monoxide concentrations were generally lower when the boat was moving than when stationary. The highest concentrations were at the stern, and the lowest were inside the boat.(4,5) The two boat engines equipped with the prototype engine control systems and catalysts that were evaluated during this current study produced relatively low ambient CO concentrations that were substantially less than the CO concentrations found in previous NIOSH studies. The data collected during this study indicated CO concentrations below occupational exposure limits, and most of the average CO data indicated concentrations below health criteria relevant to the general public. The purpose of the SWRI research was to reduce nitrogen oxide and hydrocarbon emissions using closed-loop control engines and catalysts. While CO in the engine exhaust gases was not specifically targeted for reduction, a substantial reduction in CO was observed on and around these boats compared with the boats evaluated in the above-mentioned previous studies. Specifically targeting CO by modifying the control system may result in further reductions in CO concentrations. The engines equipped with the prototype catalysts evaluated during this study can potentially provide a safer environment for the boating public. This studywas not designed to compare CO emissions from engines with catalyst to the same engines without the catalyst. Additional studies would be warranted to compare CO data collected on engines with and without a catalyst to quantify how much the catalyst reduces CO.